Cerium oxide is a highly functional ceramic material that is widely used in catalysts, fluorescent substances, cosmetics, polishing agents, and the like, and has been recently spotlighted as an abrasive for use in an STI (Shallow Trench Isolation) process of a semiconductor device, and as an optical glass polishing agent.
Such cerium oxide can be generally prepared by a liquid-phase method, a gas-phase method, or a solid-phase method.
The liquid-phase method is a method of preparing cerium oxide directly from a trivalent or tetravalent cerium salt starting material by the addition of a pH adjuster such as ammonia. This method is advantageous in that raw material and equipment costs are low. However, a reaction between starting materials easily occurs from a nucleation step, making it difficult to control particle growth.
The gas-phase method is a method of preparing cerium oxide directly by vaporizing a cerium metal salt precursor and combining the vaporized precursor with oxygen, and is subdivided into a flame combustion decomposition method, a gas condensation decomposition method, a plasma decomposition method, a laser vaporization method, etc. However, this method has a difficulty in large scale production because a cerium metal salt precursor and equipment are expensive. Thus, studies on this method are still under progress.
Meanwhile, the solid-phase method is a method of preparing cerium oxide from a precursor material through a sintering process at a high temperature, and this method has been actively studied. As the precursor, cerium carbonate-based compounds are widely used, and the shape and size thereof greatly influence the properties and shape of cerium oxide prepared therefrom, thereby affecting physical properties such as diameter of abrasive particle and shape, as well as polishing rate, flatness, or generation of scratches during the CMP process for semiconductor devices. Therefore, in order to control the properties or shape of cerium oxide within the desired ranges, there is a need for a method of preparing cerium carbonate-based compounds to easily control their type or shape.
Conventionally, there have been methods for preparing cerium carbonate-based compounds using cerium salts such as cerium nitrate and precipitants such as urea. However, these synthetic methods are problematic in that a washing step is additionally required for removing organic by-products generated by the use of precipitants and a large amount of waste solution including ammonium ions can be generated even though the washing step is performed, and thus various tube lines in a reactor become clogged and operation of a stabilizer and a pressure gauge can be deteriorated.
In addition, the cerium salts such as cerium nitrate are prepared through a complicated process such as crystallization, solubilization, or purification after dissolution of cerium salt precursors in various acids, and are also expensive, resulting in inefficiency and economic disadvantage in the preparation process of cerium oxide. Moreover, when cerium oxide is prepared using cerium carbonate prepared by such synthetic method, the obtained cerium oxide may not manifest desired physical properties such as polishing property.